Endometrial cancer (cancer of the lining of the uterus) is the most common cancer of the female reproductive tract, with about forty thousand new cases in the United States each year. A number of common genetic alterations driving the initial steps of endometrial cancer progression have been documented, such as inactivating mutations of the tumor suppressor gene PTEN. However, PTEN mutations and most other well- documented genetic lesions in endometrial cancers represent early steps driving the formation of benign hyperplasias that remain confined to the uterus. In contrast, little is known about the molecular steps promoting the progression of hyperplasias to truly invasive, lethal cancers that can spread throughout the body. Here we propose to build upon a foundation of prior research carried out in our laboratory. Specifically, we have demonstrated that inactivation of the LKB1 tumor suppressor drives the formation of highly invasive cancers in both mice and humans. The functions of LKB1 are not entirely understood, but it is known to act via its direct target AMPK to inhibit mTOR, a master regulator of cell growth. LKB1 inactivation thus leads to increased mTOR activity, which in turn promotes increased cell proliferation and cancer. Our goals are to 1) gain a more detailed view of the biological impact of LKB1 loss upon endometrial cells and invasive growth; 2) further define the molecular mechanisms by which LKB1 is inactivated in endometrial cancer; 3) study the cooperation of LKB1 with other endometrial cancer genes through the use of an in vivo genetic model system, and thereby also develop more refined genetic models of endometrial cancer; and 4) explore the use of these genetic model systems to validate an mTOR inhibitor as a pathway-specific therapy against endometrial cancer. These interrelated goals will take advantage of unique reagents and approaches including a validated endometrial Cre deletor line that enable gene targeting specifically within endometrial epithelial cells. These studies will also take advantage of a previously developed bank of human endometrial tumor specimens. These studies will lead to insights into the biological and genetic basis of endometrial cancer, create significant opportunities to develop predictive DNA-based or immunohistochemical tests for prognosis, and may someday lead to the development of improved, targeted therapies to treat or prevent endometrial cancer formation and spread.